The present invention relates to a novel process for carrying out alkylation and acylation reactions in high yield and high selectivity. More specifically the present invention relates to heterogeneous catalysis of Friedel-Crafts alkylations and acylations in a continuous or semi-batch system under supercritical or near-critical conditions.
Although Friedel Crafts alkylations and acylations are industrially important they present various problems for bulk manufacture. The catalysts generally used are Lewis acids such as AlCl3, FeCl3 and TiCl4 or strong protic acids such as hydrofluoric acid or sulfuric acid. All these catalyst present significant health, safety and environmental problems. This is also true of the solvents which are often used e.g. nitrobenzene or chlorinated solvents. In conventional Friedel Craft acylation there is also the problem of disposing of the spent catalyst sludge.
The use of supercritical fluids as a reaction media for carrying out Friedel Crafts alkylation of naphthalene with a zeolite catalyst is known (JP06065112A, JP04247045A2) but the yields are extremely low (less than 5%).
Also, the use of supercritical water as a homogeneous catalyst in a continuous process has been used for mixed alkylation to raise the octane number of gasoline (DE4342501 A1). Although high yielding, this process requires high temperature (xe2x89xa7400xc2x0 C.) and high pressures (25-100 MPa) and such a process would be of little use in selective alkylation for the manufacture of fine organic compounds.
The use of microporous crystalline catalysts such as zeolites, clays and inorganic oxides has also been used to give mixed products in the alkylation of alkenes with isoparaffins (WO 94/03415) for isoparaffin-olefin alkylation.
None of the aforementioned processes would be of benefit on an industrial scale due to either their low yield or low selectivity. The latter two citations are in any case only applicable to gaseous alkylating agents.
The most common industrial use in the fine organic industry of Friedel-Crafts reactions is the alkylation of aromatics, often using an alkene as the alkylating agent, with an acid catalyst.
Accordingly, there is a need for a high yield route to alkylated or acylated aromatic substrates. Desirably, such a route would also allow selective alkylation or acylation of aromatic substrates.
According to the present invention, there is provided a method of performing an acylation reaction on an aromatic substrate wherein an intimate mixture of the substrate and an alkylating or acylating agent is exposed under supercritical or near-critical conditions to a catalyst which includes a source of acid.
In an embodiment, the catalyst is an acid catalyst which is a Lewis acid, a sulfonic acid, an acidic resin, a zeolite, a modified zeolite, a metal oxide, a clay or a mixed oxide.
It has been found that supercritical fluids not only give an environmental benefit with regard to such processes but also provide a significant rate enhancement compared with conventional solvent systems. Surprisingly, supercritical or near-critical reactions can also be carried out with increased selectivity on a continuous or semi-batch process when using the appropriate catalyst such as those of the present invention. The present invention thus relates to both continuous and semi-batch processes, in contrast with conventional batch-type processes, performed under supercritical or near-critical conditions. Continuous processes have the advantage over such conventional processes that the xe2x80x9cdown-timexe2x80x9d of the apparatus is minimised and that the amounts of waste solvent and unconsumed reactants (which are associated with each complete batch of a conventional batch process) are minimised. Continuous processes in accordance with the invention are thus advantageous over prior art processes.
Using the method of the present invention an acidic resin, a supported Lewis acid catalyst, a sulfonic acid catalyst such as that known as Deloxan.(copyright) ASP 1/7 (acylsulfonic acid catalyst on a polysiloxane support ex. Degussa), or one of the other types of catalyst mentioned above results in the formulation of alkylated aromatic substrates in high yield and selectivity. Known Lewis acid catalysts suitable for Friedel-Crafts alkylations and acylations include AlCl3, FeCl3 and TiCl4.
The choice of catalyst may influence whether or not a particular product is formed selectively (where the possibility of more than one product exists) and thus the catalyst may be selected according to the product or products desired from the reaction. It is envisaged that, where appropriate,da combination of two or more suitable catalysts could be used. However, the use of a single catalyst is preferred. Alternatively, or additionally, selectivity may be controlled by independently varying one or more of the temperature, pressure, flow rate (in the case of a continuous process) and concentrations of the reactants.
The present invention also represents the first acylation which has been achieved under supercritical conditions. The acylation reactions according to the present invention are performed using similar conditions as for the alkylation reactions.
Reactions have been carried out in which the supercritical or near-critical fluid (for example, propene) is both the alkylating agent and the solvent. Likewise, the aromatic substrate could function as both the supercritical or near-critical fluid and the reactant. Equally, it is possible for the alkylating reagent and/or the aromatic substrate to be dissolved in a non-reacting supercritical or near-critical fluid such as CO2 or propane. This latter technique is useful for reducing the excess of alkylating agent used and also in cases where the alkylating agent is not a gas at standard temperature and pressure. In such a case the alkylating agent and substrate can be pre-mixed or added separately and dissolved in the non-reacting supercritical or near-critical fluid before passing through the reactor.
The method of the present invention allows a wider range of alkylating agents to be used (such as alkyl halides, alcohols, alkynes, esters, ethers, aldehydes and ketones as well as alkenes and alkanes) than has been used thus far in Friedel-Crafts alkylation. Similarly, for Friedel-Crafts acylation reactions, a wider range of acylating agents may be used than is the case in conventional acylation reactions. Thus carboxylic acids and derivatives thereof e.g. acid anhydrides, esters and acyl halides can be used.
The method of the present invention has the further advantage of being more gentle with the reactants than is the case with conventional reactions so that certain types of substrates which cannot be treated by the conventional acylation reaction may now be acylated. For example, whereas alkyl phenol ethers often suffer ether cleavage and form the substituted alkyl phenol such an undesirable reaction is not observed when the method of the invention is used.
Conventionally the two limiting factors for Friedel-Crafts reactions are the mass transport effect and the reaction rate effect. The use of supercritical fluids in the process of the present invention overcomes the mass transport effect and allows the reaction to be controlled by the reaction kinetic effect. Surprisingly it has been found that kinetic control of the reaction is relatively straightforward and yields greater selectivity in the reaction. Indeed in the alkylation it is possible to select conditions to give mono, di or tri-alkylated products and in the case of mono-alkylated products it is possible by choice of reactor length and catalyst loading to give significant regioselective control of the reaction. We have also found that each increase in pressure of 50 bar gave an increase of approximately 5% in conversion in certain cases. Also surprising was the observation that alkylation of mesitylene with isopropanol could be readily accomplished in 50% yield to give only the mono-product despite literature reports which doubt the ability to form this product by Friedel-Crafts methods (Chem.Ber. 120, 123, 1987).
The reaction will operate in the fluid at temperatures and pressures below the supercritical point of the fluid being used as the solvent, provided that the density of the fluid is sufficient to ensure that the starting materials (reactants) are substantially in a single phase. These conditions are hereafter referred to as being near-critical. Usually, however, the conditions employed will be supercritical.